Change of State and Latent Energy
During a phase change, the temperature of a substance does not increase or decrease. This is because the thermal energy during a phase change goes into breaking the intermolecular bonds between particles rather than increasing the kinetic energy of the particles. The latent heat is the energy released or absorbed during a change of state. ‘latent’ means hidden or unseen.
The latent heat is calculated using the formula:
Q=mL
where:
Q = the heat energy transferred in Joules (J)
m = mass (kg)
L = the latent heat (Jkg−1)
The value of L is dependent on the substance being considered and also the phase change. It takes a lot more energy to change a liquid to a gas than it does a solid to a liquid. This means the latent heat, L, for water to steam will be higher than for ice to water.
When considering a phase change from solid to liquid (melting), L is described as the latent heat of fusion ({ L }_{ fusion }). When considering a phase change from liquid to a gas (boiling), L is described as the latent heat of vaporisation ({ L }_{ vaporisation }).
Consider water:
- Latent heat of fusion ({ L }_{ fusion }) = 3.34×105 Jkg−1
- Latent heat of vaporisation ({ L }_{ vaporisation }) = 22.5×105 Jkg−1
Example 1:
How much energy is required to melt a 5kg block of ice?
Answer:
using: Q=mL
where:
m = 5 kg
{ L }_{ fusion } = 3.34×105 Jkg−1
Q=5\times (3.34\times { 10 }^{ 5 })
Q=1.67\times { 10 }^{ 6 }J
Q=1670\;kJ